This invention relates to apparatus for silver waste recovery, and more specifically, to a gravity-flow apparatus for use in the recovery of silver from photo processor waste streams.
A number of devices employing containers for recovering silver from photo processor waste streams such as spent fixing solutions from photographic paper and film processes are known in the industry. These devices generally pass the solution containing silver salts through a metal filter. The metal utilized in the metal filter is selected due to its electromotive force series characteristic being higher than that of silver. As the silver salts pass through the metal filter, a chemical replacement action causes the silver to be deposited on the metal filter. When the supply of metal on the metal filter for exchange is exhausted, the contents of the container, especially the used metal filter, are processed at a refinery to recover the silver left therefrom.
In order to safeguard the environment, it is important to be able to control the release of silver-containing solutions into the environment. Known sources of silver containing solutions have traditionally been treated to remove the silver salts from these solutions prior to disposing of the remaining solution into the environment. Attempts have been made, in the prior art, to fashion silver recovery systems that remove a large quantity of silver from the solution prior to the disposal of the solution. Some of these prior art methods unfortunately leave a relatively large silver component in the solution after the solution passes through the silver recovery systems. Currently the EPA has a requirement that waste water have no greater than five parts per million silver content. Additionally, it is also possible that city and state clean water requirements may be more strict. Accordingly, many users of silver containing solutions are in need of an effective silver recovery system.
U.S. Pat. No. 5,837,188 issued to Peterson on Nov. 17, 1998, discloses a metal filter utilized in conjunction with a non-metallic mesh material positioned about and to closely adjacent to an exterior surface of the filler filter material. This patent discusses at least some of the known prior art silver recovery systems. Some of these prior art silver recovery systems utilize steel wool as a metal filter. Others utilize large openings in screen wire permitting solution to bypass the filter material and exit the vessel with silver remaining in the solution. Others utilize solution flow from the bottom of a tube upward through metal shavings and a bed of neutralizing material. While still others utilize a mesh bag to contain the silver material when the core is moved from the interior bag. Finally, others utilize fluid deflectors and baffles to increase the length of fluid flow through the filter.
These prior art silver recovery systems suffer from a number of perceived problems. First, the affluent of the silver recovery systems exits the units after passing last through the metal filter. Accordingly, if any silver particulate breaks off on the metal filter, the silver will likely end up passing out of the unit as effluent. Secondly, if the capacity of the metal filter or filler material is close to the end of its useful life, the user may have trouble visually inspecting to see the amount of useful life left by the unit.
Although some prior art systems have employed a mesh bag around a silver recovery filter, a need exists to provide a more suitable catch for particulate matter in the solution. In the prior art, external portions of the mesh bag may become fouled with particulate and perhaps pass to the outlet and into the environment.
If the filter material of these prior art systems is depleted, then the silver-containing effluent will pass directly to the outlet. As such, the silver-containing effluent may flow outwardly of the system without reacting with the filter material.